Measuring devices, as for example, vortex flow meters are known in prior art for measuring flow velocity of liquids or gases in pipelines. Further, there are known so-called multivariable vortex meters which offer a couple of advantages over regular vortex flow meters. Specifically, these multivariable vortex meters are temperature and pressure compensated, thereby offering more precise results and enabling direct measurement of mass flow in fluids flowing through a pipeline.
Basically, a multivariable vortex meter may be configured to measure flow, relative pressure and temperature, for example, by means of a 3-in-one sensor with a microelectromechanical system chip (MEMS). This sensor chip has two measuring components: 1. A Wheatstone bridge of four resistors with piezo-resistive effect at a pressure sensitive membrane. If a differential pressure is applied, the membrane will bend, and the four resistors in the Wheatstone bridge are subject to mechanical stress and will change resistance with two resistors decreasing resistance and two resistors increasing resistance. Under application of a DC voltage, the Wheatstone bridge will yield a pressure dependent voltage output. 2. A resistor with a Temperature Coefficient of Resistance (TCR) changing due to the temperature of the MEMS.
In order to measure flow, relative pressure and temperature of a fluid flowing through a pipeline by means of only one MEMS element, the sensor may only have one open port at the positive MEMS side, as used in relative pressure sensors. At the negative pressure side, atmospheric pressure is applied from the inside of the sensor housing.
However, for measuring flow by means of vortex meters, usually a two-port sensor is implemented being arranged downstream of the bluff body. The velocity of the vortices causes small pressure changes, and when passing the two-port sensor, the latter will measure a small differential pressure. While vortices will pass the sensor in an alternating manner with respect to the positive and negative pressure ports, the signal measured by the sensor will be a small sine signal comprising some noise from hydraulic or other noise sources. The number of the vortices generated per second is proportional to the flow so that the flow algorithm has to determine the frequency of the sine and multiply it with a factor to obtain a value for the flow in the desired unit.
The use of two-port sensors in vortex flow meters has the ad-vantage that a rather good sine signal comprising little noise is obtained from the sensor. With low flow, the sine amplitude will have very low amplitude, but the relative pressure (namely, pressure related to atmospheric pressure) in the vortex tube will comprise hydraulic noise as harmonic noise components and/or large and rapid pressure deviations. Thus, there is a disadvantage with respect to the configuration described above that the two-port pressure sensor is only sensitive to differential pressure but not to relative pressure, because the two-port solution effectively protects the sine signal from being altered by the hydraulic noise of the relative pressure.
Nevertheless, the above described two-port solution is able to measure both flow and temperature. In order to be able to additionally measure relative pressure by means of the above mentioned MEMS, the two-port sensor housing has to be replaced by a one-port sensor housing.
However, when using a one-port housing, the sensor will also be sensitive to hydraulic noise emanating from the hydraulic relative pressure noise. Thus, the resulting sine curve will be modified or distorted due to that noise. In fact, since the characteristics and magnitude of the hydraulic noise differ from application to application, and even will differ over time within the same application, this unpredictable noise poses a severe problem to measuring vortex flow with only one pressure port.
Also other measuring devices, as for example, thermal flow meters comprising a plurality of temperature sensors for measuring a thermal profile around a heated body have to deal with the problem of unpredictable noise. With respect to such applications, it is necessary to measure very small temperature differences. Noise, for example, from an electronic circuit, can be problematic as well as noise caused by the medium.